U.S. patent number 6,219,550 [Application Number 09/171,418] was granted by the patent office on 2001-04-17 for handover in a mobile communication system with adjustable error correction.
This patent grant is currently assigned to Nokia Telecommunications Oy. Invention is credited to Risto Aalto, Mikko Kanerva, Juha Rasanen, Jari Vainikka.
United States Patent |
6,219,550 |
Kanerva , et al. |
April 17, 2001 |
Handover in a mobile communication system with adjustable error
correction
Abstract
The invention relates to a handover method in a mobile
communication system in which error correction of a radio signal
can be arranged with different protection levels. The method
comprises measuring the signal level and/or quality of neighbouring
base stations (BTS) at a receiver of a mobile station (MS),
comparing the measurement results thus obtained and other variables
of the connection with handover criteria, and performing a handover
from the source cell to the target cell, when the pre-set handover
criteria are met. The method of the invention is characterized by
determining at least one error connection possible for the
connection in a potential target cell for the handover, determining
at least one handover criterion on the basis of the determined
error correction of the potential target cell, and setting the
error correction of the connection in the target cell (BTS2) as
said error correction of the base station of the target cell. The
invention further relates to a mobile communication system for
performing handover.
Inventors: |
Kanerva; Mikko (Helsinki,
FI), Vainikka; Jari (Vantaa, FI), Rasanen;
Juha (Espoo, FI), Aalto; Risto (Riihimaki,
FI) |
Assignee: |
Nokia Telecommunications Oy
(Espoo, FI)
|
Family
ID: |
8548226 |
Appl.
No.: |
09/171,418 |
Filed: |
March 5, 1999 |
PCT
Filed: |
February 11, 1998 |
PCT No.: |
PCT/FI98/00124 |
371
Date: |
March 05, 1999 |
102(e)
Date: |
March 05, 1999 |
PCT
Pub. No.: |
WO98/37720 |
PCT
Pub. Date: |
August 27, 1998 |
Foreign Application Priority Data
Current U.S.
Class: |
455/436;
455/226.1; 455/525 |
Current CPC
Class: |
H04W
36/26 (20130101); H04W 36/30 (20130101) |
Current International
Class: |
H04Q
7/38 (20060101); H04Q 007/20 () |
Field of
Search: |
;455/437,438,525,436,439,443,444,422,67.1,226.1,226.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
454 638 A1 |
|
Oct 1991 |
|
EP |
|
92/12602 |
|
Jul 1992 |
|
WO |
|
95/28808 |
|
Oct 1995 |
|
WO |
|
96/26621 |
|
Aug 1996 |
|
WO |
|
96/38997 |
|
Dec 1996 |
|
WO |
|
Primary Examiner: Urban; Edward F.
Assistant Examiner: Gesesse; Tilahun
Attorney, Agent or Firm: Pillsbury Winthrop LLP
Parent Case Text
This application is the national phase of international application
PCT/F198/00124 filed Feb. 11, 1998 which designated the U.S.
Claims
What is claimed is:
1. A handover method for handing over a mobile station radio signal
call connection from a serving base station to a neighboring base
station of a mobile communication system, the system having a
capability of providing error correction of the radio signal with
different protection levels, the base stations each emitting a
signal, said method comprising:
measuring a characteristic of the signal emitted by each base
station at a receiver of the mobile station;
determining at least one possible error correction for a call
connection to the neighboring base station;
determining at least one first handover criterion on the basis of
the determined possible error correction;
comparing the signal measurements obtained in said measuring step
with handover criteria including the determined first handover
criterion;
performing a handover from the serving base station to the
neighboring base station when the comparing step indicates that the
signal measurement for the neighboring station meets the handover
criteria; and
setting the error correction for the call connection as the error
correction of the neighboring base station.
2. The method of claim 1 wherein the signal characteristic of the
signal emitted by the neighboring base station is variable about a
normal value, and the possible error correction is variable about a
normal value, and said step of determining at least one first
handover criterion comprises:
when the possible error correction for a call connection to the
neighboring base station is weaker than normal, setting the first
handover criterion to require the signal characteristic of the
signal emitted by the neighboring base station to be higher than
normal; and
when the possible error correction for a call connection in the
neighboring cell is more efficient than normal, setting the first
handover criterion to require the signal characteristic of the
signal emitted by the neighboring base station to be lower than
normal.
3. The method of claim 2 wherein said step of determining at least
one first handover criterion further comprises:
determining a normal handover criterion based on the normal signal
characteristic value; and
using the normal handover criterion as the first handover criterion
when the possible error correction has its normal value, and
wherein
said step of setting when the possible error correction for a call
connection to the neighboring base station is weaker than normal is
carried out to increase the first handover criterion from the
normal handover criterion; and
said step of setting when the possible error correction for a call
connection to the neighboring base station is more efficient than
normal is carried out to decrease the first handover criterion from
the normal handover criterion.
4. A mobile communication system including a mobile station, a
serving base station and a neighboring base station, the system
being operative for handing over a mobile station radio signal call
connection from the serving base station to the neighboring base
station, and the system having a capability of providing error
correction of the radio signal with different protection levels,
the base stations each emitting a signal, said system
comprising:
means for measuring a characteristic of the signal emitted by each
base station at a receiver of the mobile station;
means for determining at least one possible error correction for a
call connection to the neighboring base station;
means for determining at least one first handover criterion on the
basis of the determined possible error correction;
means for comparing the signal measurements obtained in by the
means for measuring step with handover criteria including the
determined first handover criterion;
means for performing a handover from the serving base station to
the neighboring base station when the result produced by the means
for comparing indicates that the signal measurement for the
neighboring station meets the handover criteria; and
means for setting the error correction for the call connection as
the error correction of the neighboring base station.
5. The mobile communication system of claim 4 wherein the error
correction is channel coding.
Description
FIELD OF THE INVENTION
The invention relates to a handover method in a mobile
communication system in which error correction of a radio signal
can be arranged with different protection levels. The method
comprises measuring the signal level and/or quality of neighbouring
base stations and a serving base station at a receiver of a mobile
station, comparing the measurement results thus obtained and other
variables of the connection with handover criteria, and performing
a handover from the source cell to the target cell when the
handover criteria are met.
The invention also relates to a mobile communication system for
performing handover.
BACKGROUND OF THE INVENTION
In cellular mobile communication systems, a radio coverage area is
implemented with a plurality of slightly overlapping radio cells.
When a mobile station moves from one cell to another, handover to a
new radio cell is performed on the basis of predetermined handover
criteria. The aim is to perform handover in a manner that disturbs
an ongoing call as little as possible. Handover is normally
performed on the basis of radio path criteria, but it may also be
performed for other reasons, e.g. to divide the load, or to reduce
transmission powers, or when a mobile station has moved too far
from the base station, in which case the propagation delay of the
radio signal becomes too great.
FIG. 1 of the appended drawings shows a simplified block diagram of
the pan-European GSM mobile communication system. A mobile station
MS is connected over a radio path to a base transceiver station
BTS, in the case of FIG. 1 to BTS1. A base station system BSS
consists of a base station controller BSC and base stations BTS
controlled by the BSC. A plurality of base station controllers BSC
usually operate under the control of a mobile services switching
centre MSC. An MSC communicates with other MSCs and, through a
gateway mobile services switching centre GMSC, with a public
switched telephone network. The operation of the entire system is
controlled by an operation and maintenance centre OMC. The
subscriber data of a mobile station MS are stored permanently in a
home location register HLR of the system and temporarily in the
visitor location register VLR in the area of which the MS is
located at a given moment.
A mobile station MS and the serving base station BTS1 continuously
measure the signal level and quality of the radio connection for
example to determine the need for handover. The MS measures the
signals of the serving base station BTS1 and the base stations BTS
that are closest to its location area for instance to select a
suitable target cell for handover. In the GSM mobile communication
network, for example, an MS may simultaneously measure the signal
level of both the serving base station and up to 32 other base
stations. Via the serving base station BTS1, the MS is informed of
the neighbouring cells it should measure. The measurement results
of each cell are identified on the basis of the combination of a
base station identity code BSIC and the frequency of the broadcast
control channel BCCH.
The mobile station MS sends the measuring results regularly as a
report message through the serving base station BTS1 to the base
station controller BSC. A report message contains the measurement
results of the serving base station and up to six best neighbouring
base stations. Handover from a serving cell to a neighbouring cell
may take place, for example, when the measurement results of the
mobile station/base station indicate a low signal level and/or
quality and a higher signal level is achieved in the neighbouring
cell, or when a neighbouring cell allows communication at lower
transmission powers. In addition, a handover to the most suitable
neighbouring cell is performed when the serving base station is
overloaded, or when an MS has moved too far from the serving base
station BTS, or if it is necessary to change the base station for
some other reason. The selection of the target cell for handover is
influenced, for example, by the signal level and/or load of the
target cell. In order for the stability of the mobile communication
network to be ensured, the measurement results and parameters used
in handover are averaged over a given period of time. The averaging
process makes handover less sensitive to measurement results
distorted by instantaneous interference or fading.
The base station controller BSC makes the decisions relating to
handover. If the target cell is controlled by another BSC, the
handover may be performed under the control of the MSC. Another
possibility is that handover decisions are always made in a
centralized manner in the MSC. If necessary, the BSC gives a
handover command to the MS through the BTS.
In a mobile communication system implemented by code division
multiple access (CDMA) technology, handover performed in the manner
described above is called hard handover. In addition, CDMA systems
can use so-called soft handover, in which a mobile station, during
a handover, may be simultaneously connected to the network through
several base stations. When one of these base stations proves to be
better than the others on the basis of its signal, the connections
of the mobile station with the other base stations are released,
and the call is continued only through the best base station. Soft
handover prevents recurring handovers between base stations when a
mobile station is located on the periphery of cells.
Transmission errors which deteriorate the quality of a transmitted
signal occur on the transmission path when speech or data is
transmitted in a digital telecommunication system. Transmission
errors occur on the transmission path when a signal is disturbed,
for example, on account of multipath propagation, an interfering
signal or high background noise level. Error correction of a
digital signal to be transmitted, e.g. channel coding and/or
retransmission, is used for improving the quality of the
transmission and the tolerance of transmission errors. In channel
coding, repetition is added to the original bit string of encoded
speech or data by error correcting bits calculated from the
original signal. In the receiver, the channel coding is decoded in
a channel decoder, whereby the signal errors that have occurred
during the transmission can be detected or even corrected by means
of the correcting bits. Retransmission is used for correcting
transmission errors either independently or, for example, in
addition to channel coding: the errors in a channel coded
transmission are corrected by retransmission of distorted frames.
When the quality of the connection deteriorates, the number of
erroneous and lost frames grows, and thus also the number of
retransmissions grows.
Channel coding increases the number of bits to be transmitted. In
the GSM mobile communication system, for example, error correcting
bits with a transmission rate of 9.8 kbit/s are added to a
full-rate 13 kbit/s speech signal, whereby the total transmission
rate is 22.8 kbit/s. The level of the protection provided by
channel coding is arranged according to the need. If a large number
of data are to be transmitted fast, the amount of channel coding is
reduced to allow more payload to be transmitted on the transmission
channel. Channel coding may be arranged either to both detect
errors occurred during the transmission and correct them or to
merely detect them. In the GSM system, bits to be transmitted are
divided according to their importance into different classes, in
which channel coding is provided at a predetermined level. The
different elements of the mobile communication system may limit the
selection and implementation of the channel coding provided for a
connection. A mobile station may, for instance, support only
certain channel codings. Furthermore, the protection level of the
provided channel coding depends on the capability of the base
station and other network elements to employ different channel
codings.
A problem with the handovers of the prior art is that the quality
of the radio connection after handover is not always sufficient for
a connection using weak channel coding. Another problem is that
since network planning is usually intended for channels using
normal channel coding, a handover is not performed to a target cell
whose signal level and/or quality is too low for a connection using
normal channel coding but sufficient for a connection employing
more efficient channel coding.
BRIEF DESCRIPTION OF THE INVENTION
An object of the present invention is to provide an optimal way of
selecting the target cell for a handover in an environment where
error corrections of several different levels are used.
This new type of handover is achieved with a method of the
invention, which is characterized by determining at least one error
correction possible for the connection in a potential target cell
for the handover, determining at least one handover criterion on
the basis of the determined error correction of the potential
target cell, and setting the error correction of the connection in
the target cell as said error correction of the base station of the
target cell.
Another object of the invention is provide a mobile communication
system where error correction of a radio signal can be arranged
with different protection levels, said system being arranged to
measure the signal level and/or quality of neighbouring base
stations and a serving base station at a receiver of a mobile
station, compare the measurement results thus obtained and other
variables of the connection with handover criteria, and perform a
handover from the source cell to the target cell, when the pre-set
handover criteria are met. According to the invention, the mobile
communication system is characterized in that by being arranged to
determine at least one error correction possible for the connection
in a potential target cell for the handover, determine at least one
handover criterion on the basis of the determined error correction
of the potential target cell, and set the error correction of the
connection in the target cell as said error correction of the
target cell.
The invention is based on the idea that, when a target cell is
selected, the effect of error correction on the signal level
required on the radio connection is taken into account.
In the handover method of the invention, the selection of the
target cell for a handover is affected by the protection level of
the error correction, preferably channel coding, of a potential
target cell; the handover criterion is determined on the basis of
this level. According to a preferred embodiment of the invention,
the value of the handover criterion is increased from the normal
when the error correction offered by the target cell is weaker than
normal, and decreased from the normal when the error correction
offered by the target cell is more efficient than normal. When a
handover is to be performed, for example, because of overload in
the source cell, it is checked whether the handover criterion of
the invention and other handover criteria optionally set by the
operator are met in the potential target cell; the handover is
performed to the target cell whose base station signal best meets
the pre-set handover criteria. At the same time, the error
correction of the connection is set to be as determined in
connection with the handover procedure of the target cell.
An advantage of such a handover is that it is ensured that the
quality and/or level of the signal of the target cell is sufficient
even for a radio connection using weak error correction to prevent
calls from being unnecessarily lost.
Another advantage of the handover of the invention is that it can
be performed to a target cell of a lower level than normal on
account of suitable error correction.
Yet another advantage of the handover of the invention is that a
network planned for normal error correction can offer optimal
handover even to mobile stations whose error correction is
implemented at a protection level different from the normal.
A further advantage of the handover of the invention is that a
change in the protection level of error correction during handover
ensures that the quality of the radio connection to be handed over
remains sufficiently good.
Still another advantage of the claimed handover is that it reduces
the probability of a call being lost on a radio connection
implemented with weaker error correction than normal.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following, the invention will be described in greater detail
with reference to the accompanying drawings, in which
FIG. 1 is a block diagram of the structure of a mobile
communication system,
FIG. 2 shows an example of handover in a cellular network, and
FIG. 3 is a flow chart of the preferred embodiment of the handover
method of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention can be applied in any cellular mobile
communication system. In the following, the invention will be
described in greater detail with reference, by way of example, to
the pan-European digital mobile communication system GSM. FIG. 1
illustrates the above-described structure of a GSM network in a
simplified manner. For a more accurate description of the GSM
system, reference is made to the GSM recommendations and "The GSM
System for Mobile Communications" by M. Mouly and M-B. Pautet,
Palaiseau, France, 1992, ISBN: 2-9507190-0-7.
The present invention can be applied to different kinds of channel
coding. An example of channel coding is the convolution coding
which is used on a traffic channel of the GSM system and defined in
GSM Recommendation 05.03. The efficiency of convolution coding can
be indicated with the convolution code ratio X/Y, which means that
X data bits are represented by Y code bits in channel coding. On a
full-rate GSM traffic channel, the convolution code ratios are 1/2
(buffered), 1/3 and 1/6 for data rates of 9.6 kbit/s, 4.8 kbit/s
and 0.3 to 2.4 kbit/s, respectively. A full-rate speech channel
employs 1/2 channel coding. The most efficient channel coding is
1/6, the second most efficient is 1/3, and the weakest is 1/2.
As stated above, a problem with handovers of the prior art is that
the quality of the radio connection after a handover is not always
sufficient for a connection using weak channel coding. Another
problem is that since network planning is usually made for channels
using normal channel coding, a handover is not performed to a
target cell whose signal level and/or quality is too low for a
connection using normal channel coding, but sufficient for a
connection employing more efficient channel coding.
In the following, the invention will be described more generally
without reference to any specific type of channel coding.
FIG. 2 shows an example of a handover when a mobile station MS
moves within the area of base station BTS1 in a cellular network.
The area of the base station in which the MS is located before the
handover will be hereafter called a source cell, and the area of
the base station to which the handover is performed will be called
a target cell. FIG. 2 illustrates the coverage areas C1 and C2 of
the base stations BTS1 and BTS2; within these coverage areas the
quality of a radio connection using normal error correction, e.g.
channel coding, usually remains sufficiently high to prevent calls
from being lost. A radio connection using error correction weaker
than normal requires a better signal than normal to achieve
sufficient connection quality; correspondingly, error correction
that is more efficient than normal renders it possible to achieve
sufficient connection quality even with a signal that is weaker
than normal. This effect of error correction (in this case the
effect of channel coding) on the size of the coverage area of a
base station is illustrated in FIG. 2 by broken lines. Line C2W
represents the effective coverage area of BTS2 with channel coding
that is weaker than normal, and line C2E indicates the effective
coverage area of BTS2 with channel coding that is more efficient
than normal. Channel coding that is weaker than normal will
hereafter be called weak channel coding, whereas channel coding
that is more efficient than normal will be called powerful channel
coding. In the case of GSM, normal channel coding refers herein to
channel coding of a full-rate channel.
When a serving base station wishes to hand an ongoing call of a
mobile station MS over to another base station (for instance on
account of overload at the serving base station, or if the MS is
located too far from the serving base station), a handover is
performed to the base station of the neighbouring cell whose signal
meets the pre-set handover criteria. In FIG. 2, a handover of the
prior art is shown to be performed at point .times.1 when the MS
moves in the direction of the arrow in the area C1 of BTS1 and
arrives at the coverage area C2 offered by the normal channel
coding of BTS2. One criterion used for triggering a handover is the
results of neighbouring base station measurement, reported by the
MS to the network. The handover of the prior art is started when
the measurement results and other parameters of the connection meet
the pre-set handover criteria. A common handover criterion for the
selection of a target cell is the signal level of a neighbouring
base station, e.g. BTS2, as compared with that of the serving base
station BTS1. When the signal level meets the pre-set handover
criterion and the other handover criteria are also met, a handover
is performed from the serving base station BTS1 to the neighbouring
base station BTS2 in question (in the case of FIG. 2 at point
.times.1, for example). In the case of a radio connection using
weak channel coding, the effective coverage area of BTS2 thus
corresponds to area C2W indicated by a broken line in FIG. 2, and
therefore a handover performed at point .times.1 is performed too
soon and the call will probably be lost.
In the following, the invention will be described in greater detail
with reference to the preferred embodiment. In this embodiment, the
error correction of a radio connection is implemented with channel
coding. One handover criterion used in the preferred embodiment is
the signal level of the target cell. FIG. 3 is a flow chart of the
preferred embodiment of the handover method of the invention. For
reasons of clarity, the handover procedure will be explained in the
following with reference to only one channel coding alternative of
one target cell. It will, however, be obvious to one skilled in the
art that the following also applies if several potential target
cells are monitored. In this case, the handover criterion of the
invention is determined separately for the channel codings of each
target cell; before a handover decision is made, each handover
criterion is checked to find out whether it is possibly met.
In step 30 of FIG. 3, a mobile station MS measures signals from the
neighbouring base stations according to the prior art. In step 31,
the unit that makes the handover decision, preferably a base
station controller BSC or a mobile services switching centre MSC,
determines the channel coding offered to the radio connection by
the possible target cell for handover (BTS2 in the case of FIG. 2),
in accordance with the invention. The method of the invention is
thus particularly well applicable even to networks where all base
stations cannot use all the different channel coding alternatives.
In step 32, the channel coding offered by BTS2 of the possible
target cell is compared, according to the invention, with normal
channel coding. If the channel coding of BTS2 of the possible
target cell is weaker than normal, the value of the handover
criterion is increased in step 33 from the normal; e.g. the
handover criterion is set such that it is met when the signal level
of the target cell is higher than -97 dBm. If the channel coding of
BTS2 of the possible target cell is not weaker than normal, it is
checked in step 34 whether the channel coding of BTS2 of the target
cell is more powerful than normal channel coding. If the channel
coding of BTS2 is more powerful than normal channel coding, the
value of the handover criterion is decreased in step 35 from the
normal; e.g. the handover criterion is set such that it is met when
the signal level of the target cell is higher than -103 dBm.
A handover is performed (step 37) from the base station BTS1 of the
source cell to the base station BTS2 of the target cell, if the
pre-set handover criterion and any other handover criteria set by
the operator are met (step 36). The handover criterion of the
invention is met, for example, if it is set in step 33 to be -97
dBm, and the measurement results of the MS show that the signal
level of BTS2 of the target cell meets this criterion. When the
handover is performed, the channel coding of the radio connection
is set to be as determined in step 31.
Certain discrete values can be stored for the handover criterion of
the invention. In this case it is possible to define a
predetermined handover criterion value for each protection level of
error correction. In steps 33 and 35 of FIG. 3, a value
corresponding to the channel coding that is being monitored is thus
set for the handover criterion. If, for example the base station of
the target cell uses weak, normal and powerful channel coding, the
values of the handover criterion can be set to be -97, -100 and
-103 dBm, respectively.
In the following, the invention will be described more closely with
reference to the example of FIG. 2. As shown in FIG. 2, BTS2 can,
in addition to normal channel coding, also offer powerful (cell
coverage area C2E) and weak (cell coverage area C2W) channel coding
to a connection. If a handover is planned to be performed to BTS2
such that powerful channel coding will be used, the handover
criterion is decreased from the normal according to the invention;
this triggers the handover, which is performed e.g. as early as at
point .times.3, provided that the other handover criteria
optionally set by the operator are met. In this example, the radio
connection between BTS2 and the MS is continued after the handover
with powerful channel coding. If a handover is planned to be
performed to the BTS2 of the target cell such that weak channel
coding will be used, the handover criterion is increased according
to the preferred embodiment of the invention. In this case, the
handover and the setting of channel coding are performed, for
instance, at point .times.2 indicated in FIG. 2. The channel coding
of the connection is then set at BTS2 as weak channel coding.
According to another embodiment of the invention, a handover is
performed as stated above with reference to the preferred
embodiment, except that, in addition to the channel codings offered
to the radio connection by the base station of the target cell, the
channel coding desired for the MS connection is also determined in
step 31 of FIG. 3. In connection with call set-up, for example, an
MS or some other network element may indicate the wishes it has
concerning the error correction or transmission rate of the
connection. If the base station of the target cell can offer the
desired/allowed channel coding and also other channel codings, the
aim is to select the desired/allowed channel coding that meets the
handover criterion as the channel coding of the target cell. This
channel coding will be used from step 32 of FIG. 3 onwards as the
channel coding determining the handover criterion of the method of
the invention and as the channel coding to be set for the
connection. If more than one channel codings meet the handover
criterion determined according to the invention, the channel coding
that has the widest margin for the handover criterion in question
will be selected as the channel coding of the target cell for the
handover.
Instead of the signal level of the target cell, used as the
handover criterion in the preferred embodiment, other embodiments
of the invention may use some other suitable handover criterion of
the prior art. In cellular systems where the quality of the signal
from neighbouring base stations is measured, this quality can be
used as the handover criterion. The signal from the neighbouring
base stations is measured in a manner suitable for each system.
The claimed target cell selection and handover to a target cell are
suitable for many different cases of handover. Load at the serving
base station, transmission delay on the radio connection, low
quality or level of the signal of the serving base station, for
example, may give rise to a need for handover and start the
selection of a target cell for a possible handover.
The protection level of the error correction used in the source
cell is not relevant to the handover method of the invention. For
example, the handover of a connection implemented with weak channel
coding in the source cell to a target cell without changing the
channel coding is performed, according to the invention, such that
the connection will use weak channel coding in the target cell. The
present handover method is suitable for changing the error
correction simultaneously with a handover. The method of the
invention is not significant in practice, if the error correction
of the connection represents the normal protection level in the
target cell. The functionality of the invention can be ignored in
the handover, and the handover can be implemented according to the
prior art.
In addition to the measurement and handover means required for a
handover of the prior art, the mobile communication system
implementing the functionality of the invention comprises means for
determining error correction possible for the connection in a
potential target cell for the handover, means for determining a
handover criterion on the basis of the error correction determined,
and means for setting the error correction as said error correction
determined during the handover. The means for performing a handover
of the invention are preferably provided at the base station
controller BSC or the mobile services switching centre MSC.
The drawings and the description relating to them are intended
merely to illustrate the inventive concept. In its details, the
handover of the invention may vary within the scope of the claims.
Although the invention has been described above mainly with
reference to channel coding, the invention can also be used with
other kinds of error correction. According to the invention, the
use of error correction in the selection of the target cell for
handover can be combined with any handover criteria of the prior
art. In addition to mobile communication systems of the TDMA type,
the invention is also applicable to other cellular mobile
communication systems, e.g. systems implemented by the CDMA
technology, particularly in the case of hard handover.
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